This invention relates to a solder paste, and more particularly to a solder paste for the soldering of electronic parts in an electronic apparatus.
Recently, solder paste has come to be widely used for mounting electronic parts such as semiconductor chips on printed circuit boards or heat sinks. Generally, solder paste contains powdered solder comprising an Sn-Pb alloy. In order to minimize the effects of heat on the parts when they are being soldered, a solder composition of 63% Sn-Pb is commonly used since it has a low melting point (183.degree. C.) compared to other Sn-Pb alloys in addition to excellent solderability. However, for parts which are subjected to higher temperatures during the operation of an electronic apparatus, such as parts equipped with a heat sink, a solder paste containing powdered solder with a higher melting point may be used, such as a 55% Sn-Pb solder with a melting point of 310.degree. C. or a 5% Ag-Pb solder with a melting point of 365.degree. C.
When an electronic part such as a semiconductor chip is soldered to a supporting member such as printed circuit board using solder paste, a suitable amount of the solder paste is applied to the printed circuit board in selected locations by a printing process such as screen printing or by blowing the solder from a dispenser. The semiconductor chip is then placed atop the solder paste. The printed circuit board with the chip resting thereon is then placed into a reflow furnace or an infrared heating apparatus so as to melt the solder paste and solder the chip to the printed circuit board. A similar method is used for soldering an electronic part to a heat sink.
However, the peeling strength of a soldered connection made using a conventional solder paste is low, and even a small shock may cause electronic parts to come loose from the supporting members. Furthermore, during the operation of an apparatus containing electronic parts which are soldered using solder paste, cracks often form in the solder which not only result in inadequate electrical contact but may also cause the parts to come loose from the supporting members.